1. Field of the Invention
The invention pertains generally to a controller for monitoring and regulating photovoltaic (PV) power systems, ranging from relatively simple stand-alone PV power systems to larger PV/fuel generator hybrid power systems. More particularly, the invention is directed to a microprocessor-based programmable controller which is specifically designed for utilization at unattended system installations in remote sites with harsh environments, and which effectively achieves optimum charging of the system batteries by the photovoltaic array and/or the auxiliary generator. The controller is specifically adapted to regulate the PV array, to control the auxiliary generator, to manage the system load, to monitor and display system parameters and to energize an alarm in response to the occurence of certain critical system conditions. The controller accomplishes the foregoing functions by means of a microprocessor having a control algorithm which is uniquely capable of being redefined to accommodate new and diverse PV system control schemes.
The stand-alone PV power system typically includes a PV array comprising an assembly of solar photovoltaic cells connected in a series parallel configuration, and a system battery comprising a block of batteries which are charged by current from the PV array to serve as the primary power source for a load. Although system efficiency dictates that the battery receive the maximum possible amount of charge, it is essential to accurately regulate the PV array in order to prevent battery overcharge with its associated excess gassing and potential destruction of the battery. In view of the fact that the battery represents a very costly and sensitive component in the PV power system, the importance of successful and precise regulation of battery charge is even further intensified.
The controller of the present invention initiates charge regulation for the battery when the system voltage reaches a predetermined level, being adapted to disconnect the array from the battery so as to allow the system voltage to fall to a more desirable level. When the battery voltage reaches a second lower setpoint, the controller is adapted to effect reconnection of the array to resume battery charging. The controller is thus capable of insuring optimum battery charging and maximization of battery life.
It is similarly important that the battery in a PV power system be protected from a state of undercharge, which can cause damaging reversal of polarity of the battery. Hence, it is essential that a control mechanism be provided for regulating the powered load so as to prohibit adverse depletion of the battery charge by the load when the PV array is unable to adequately charge the battery. The subject controller provides such a mechanism by accomplishing disconnection of the load from the battery at a predetermined system voltage in order to interrupt the battery discharge mode. The load disconnect function of the controller possesses a built-in time delay feature that prevents nuisance disconnections. Once disconnected, the load is adapted to remain disconnected until a higher system voltage setpoint is reached, at which time the controller effects reconnection of the batteries to the load.
The typical hybrid power system includes, in addition to the PV array, battery and load, a fuel consumable Thermo Electric Generator (TEG) or Diesel Electric Generator (DEG). Depending upon system design, the latter generators may be utilized to charge the system battery as a supplement to the PV array when the array is deliberately undersized, when the array fails to maintain a desired system voltage, or as a function of the ambient temperature of the system, to serve as a back-up for the PV array in the event of a system failure or to directly power the load, as dictated by design requirements.
The controller of the present invention is able to control operation of a generator to maintain the desired battery state of charge or in response to other diverse system conditions. Typically, the controller will initiate operation of a generator when the battery voltage falls to a predetermined setpoint. Once the controller has caused the generator to be turned on, the generator generally will remain on until the battery voltage, as determined by the microprocessor, reaches a higher predetermined level.
All of the control functions of the present controller are based upon the temperature compensated system voltage. Similarly, all of the controller functions are corrected for temperature by means of a coefficient incorporated into the control algorithm. The applicable compensation coefficient is adapted to be adjusted in the field to one from a range of available coefficients by setting a DIP switch. The controller thus effectively accommodates the fact that battery characteristics, including voltage, vary with ambient temperature, and age, thereby resulting in superior accuracy of control and operation.
Moreover, all of the foregoing setpoints, namely, the array disconnect and reconnect setpoints, the load disconnect and reconnect setpoints and the generator start and stop setpoints, are adapted for easy adjustment in the field over a wide range by means of a bank of DIP switches, so as to accommodate an entire spectrum of system operating parameters.
The subject photovoltaic system controller is further adapted to monitor system status, being capable of performing continuous system diagnostic functions. The controller may be provided with an annunciator panel including LED system status indicators, as well as a digital meter for reading voltages, currents and temperatures. The indicators may selectively display "normal" system status, i.e. normal charging or discharging of the battery; system voltage; array, load and battery currents; and ambient temperature. The display system is particularly designed to require only a small amount of power.
Additionally, the photovoltaic system controller is capable of providing an alarm when the system voltage falls below a field adjustable low alarm setpoint. The controller is adapted to light an alarm LED, and is able to send a signal which may activate an external alarm.
The foregoing capabilities are provided by the logic unit of the controller, wherein processor-based design provides powerful, flexible control and capability. The control algorithm for the logic unit is characterized by non-volatile memory storage to insure that system operation returns to normal following any loss of, or temporary drop in, system voltage below the minimum required for controller operation. Of particular significance is the fact that the control algorithm may be easily redefined merely by replacing the erasable programmable memory component. The controller is thus able to be adapted to standard as well as custom programming to afford a broad variety of system applications.
The controller incorporates digital circuitry which does not require periodic adjustments or alignments in order to maintain control accuracy. Also included is an emergency back-up control, comprising analog circuits, for the array and load relays in the event of loss of main controller function. Moreover, the controller's memory, logic functions, data/control bus, and input/output ports are all integral to a single circuit, thus providing advantageously for fewer parts. A further unique characteristic is that the controller derives its operating power from the system battery.
The controller is designed in conformance with commercial and military standards, and is adapted for use over long periods. The logic unit is enclosed in a heavy steel casing which is easy to install and which is able to shield against transient surges. Thus, the controller is uniquely suited to withstand the vibration and noise conditions typical of diesel generator installations, as well as the harsh physical environments typical of remote site applications. Finally, the controller is able to realize an efficiency rating better than 99% while being adapted for utilization by relatively unskilled personnel.
2. Description of the Prior Art
The prior art discloses means for controlling the photovoltaic charging of storage batteries. For example, U.S. Pat. No. 4,327,318 to Kwon et al teaches a source shedding voltage regulator which selectively sheds portions of the PV array into or from the charging system by switching means in response to the terminal voltage of the storage battery. Voltage monitoring means compares the terminal voltage to a predetermined reference potential. Logic means provides a control signal to initiate shedding.
U.S. Pat. No. 4,551,980 to Bronicki discloses a hybrid power generating system wherein a current sensor senses the current produced by a PV array and a charge level detector continuously monitors the charge level of a battery in relation to a predetermined percentage of full battery charge. A logic circuit initiates start-up of the generator when no current is being produced by the PV array.
Chetty et al, in "Microprocessor-Controlled Digital Shunt Regulator" shows a microprocessor-controlled regulator which adds or removes solar array sections to maintain the shunt current between its maximum and minimum limits.
The prior art fails to disclose a photovoltaic system controller which is characterized by reduced power requirements, is able to withstand the harsh environment of remote sites, requires little maintenance and adjustment for unattended installations, and incorporates a microprocessor having a reprogrammable memory for adapting the controller to diverse operating parameters. The prior art also does not teach or suggest a photovoltaic system controller wherein a plurality of setpoints may be provided to the microprocessor, all of the setpoints being field adjustable. The prior art does not disclose a controller which is adapted to completely monitor and regulate a stand-alone PV power system or a PV hybrid power system by regulating the PV array, managing the powered load, controlling an auxiliary generator, displaying system status and indicating the occurence of certain critical events in the system.